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== Wave energy converters == [[File:WECs-2020.png|thumb|upright=2|Generic wave energy concepts: 1. Point absorber, 2. Attenuator, 3. Oscillating wave surge converter, 4. Oscillating water column, 5. Overtopping device, 6. Submerged pressure differential, 7. Floating in-air converters.]] Wave energy converters (WECs) are generally categorized by the method, by location and by the [[power take-off]] system. Locations are shoreline, nearshore and offshore. Types of power take-off include: [[hydraulic ram]], [[Peristaltic pump|elastomeric hose pump]], pump-to-shore, [[Hydroelectricity|hydroelectric turbine]], air turbine,<ref>[https://web.archive.org/web/20060523114110/http://classes.engr.oregonstate.edu/eecs/fall2003/ece441/groups/g12/White_Papers/Kelly.htm Embedded Shoreline Devices and Uses as Power Generation Sources] ''Kimball, Kelly, November 2003''</ref> and [[Linear alternator|linear electrical generator]]. [[File:Wave energy power take-off alternatives.png|thumb|upright=1.35|Different conversion routes from wave energy to useful energy in terms or electricity or direct use.]] The four most common approaches are: * point absorber buoys * surface attenuators * oscillating water columns * overtopping devices === Point absorber buoy === This device floats on the surface, held in place by cables connected to the seabed. The point-absorber has a device width much smaller than the incoming wavelength Ξ». Energy is absorbed by radiating a wave with destructive interference to the incoming waves. Buoys use the swells' rise and fall to generate electricity directly via [[linear alternator|linear generators]],<ref name="Seabased">{{cite web|title=Seabased AB wave energy technology|work=Seabased |url=http://www.seabased.com/en/technology/seabased-wave-energy |access-date=October 10, 2017|archive-url=https://web.archive.org/web/20171010211446/http://www.seabased.com/en/technology/seabased-wave-energy|archive-date=October 10, 2017|url-status=live}}</ref> generators driven by mechanical linear-to-rotary converters,<ref name="PowerBuoy">{{cite web |title=PowerBuoy Technology |publisher=Ocean Power Technologies |url=http://www.oceanpowertechnologies.com/powerbuoy-technology/ |access-date=October 10, 2017|archive-url=https://web.archive.org/web/20171010213214/http://www.oceanpowertechnologies.com/powerbuoy-technology/|archive-date=October 10, 2017|url-status=live}}</ref> or hydraulic pumps.<ref name="CETO">{{cite web|title=Perth Wave Energy Project β Carnegie's CETO Wave Energy technology |url=https://arena.gov.au/projects/perth-wave-energy-project/|access-date=October 10, 2017|archive-url=https://web.archive.org/web/20171011072056/https://arena.gov.au/projects/perth-wave-energy-project/|archive-date=October 11, 2017|url-status=live}}</ref> Energy extracted from waves may affect the shoreline, implying that sites should remain well offshore.<ref name="Tethys">{{cite web|title=Tethys |url=http://tethys.pnnl.gov/technology-type/wave|access-date=April 21, 2014|url-status=live|archive-url=https://web.archive.org/web/20140520003234/http://tethys.pnnl.gov/technology-type/wave|archive-date=May 20, 2014}}</ref> One point absorber design tested at commercial scale by [[CorPower Ocean|CorPower]] features a negative spring that improves performance and protects the buoy in very large waves. It also has an internal pneumatic cylinder that keeps the buoy at a fixed distance from the seabed regardless of the state of the tide. Under normal operating conditions, the buoy bobs up and down at double the wave amplitude by adjusting the phase of its movements. It rises with a slight delay from the wave, which allows it to extract more energy. The firm claimed a 300% increase (600 kW) in power generation compared to a buoy without phase adjustments in tests completed in 2024.<ref>{{Cite web |last=Blain |first=Loz |date=2024-03-07 |title=Video: Wave-amplifying generator bounces twice as high as the swells |url=https://newatlas.com/energy/corpower-wavespring/ |access-date=2024-04-12 |website=New Atlas |language=en-US}}</ref> === Surface attenuator === These devices use multiple floating segments connected to one another. They are oriented perpendicular to incoming waves. A flexing motion is created by swells, and that motion drives hydraulic pumps to generate electricity. The [[Pelamis Wave Energy Converter]] is one of the more well-known attenuator concepts, although this is no longer being developed.<ref name="admin2014">{{cite news |date=21 November 2014 |title=Wave power firm Pelamis calls in administrators |url=https://www.bbc.co.uk/news/uk-scotland-scotland-business-30151276 |access-date=2024-04-13 |work=BBC News |publisher=}}</ref> === Oscillating wave surge converter === These devices typically have one end fixed to a structure or the seabed while the other end is free to move. [[Energy]] is collected from the relative motion of the body compared to the fixed point. Converters often come in the form of floats, flaps, or membranes. Some designs incorporate [[parabolic reflector]]s to focus energy at the point of capture. These systems capture energy from the rise and fall of waves.<ref name="Renewable Sea Power">{{cite journal| last1=McCormick |first1=Michael E. |first2=R. Cengiz |last2=Ertekin |title=Renewable sea power: Waves, tides, and thermals β new research funding seeks to put them to work for us |journal=Mechanical Engineering |publisher=ASME |volume=131 |issue=5 |year=2009 |pages=36β39|doi=10.1115/1.2009-MAY-4 |doi-access=free }}</ref> === Oscillating water column === [[Oscillating Water Column|Oscillating water column]] devices can be located onshore or offshore. Swells compress air in an internal chamber, forcing air through a turbine to create [[electricity]].<ref name="βgrnflea">{{cite web|title=Extracting Energy From Ocean Waves |url=http://grnflea.com/extracting-energy-from-ocean-waves/|access-date=April 23, 2015|archive-url=https://web.archive.org/web/20150815152057/http://grnflea.com/extracting-energy-from-ocean-waves/|archive-date=August 15, 2015}}</ref> Significant noise is produced as air flows through the turbines, potentially affecting nearby [[birds]] and [[marine organisms]]. Marine life could possibly become trapped or entangled within the air chamber.<ref name="Tethys" /> It draws energy from the entire water column.<ref name=":1">{{Cite web |last=Blain |first=Loz |date=2022-08-01 |title=Blowhole wave energy generator exceeds expectations in 12-month test |url=https://newatlas.com/energy/blowhole-wave-energy-generator/ |access-date=2022-08-08 |website=New Atlas |language=en-US}}</ref> === Overtopping device === Overtopping devices are long structures that use wave velocity to fill a reservoir to a greater water level than the surrounding ocean. The potential energy in the reservoir height is captured with low-head turbines. Devices can be on- or offshore. === Submerged pressure differential === Submerged pressure differential based converters<ref>{{Cite journal |last1=Kurniawan |first1=Adi |author-link2=Deborah Greaves |last2=Greaves |first2=Deborah |last3=Chaplin |first3=John |date=December 8, 2014 |title=Wave energy devices with compressible volumes |journal=Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences |volume=470 |issue=2172 |pages=20140559 |issn=1364-5021 |doi=10.1098/rspa.2014.0559 |pmc=4241014 |pmid=25484609 |bibcode=2014RSPSA.47040559K}}</ref> use flexible (typically reinforced rubber) membranes to extract wave energy. These converters use the difference in pressure at different locations below a wave to produce a pressure difference within a closed power take-off hydraulic system. This pressure difference is usually used to produce flow, which drives a turbine and electrical generator. Submerged pressure differential converters typically use flexible membranes as the working surface between the water and the power take-off. Membranes are pliant and low mass, which can strengthen coupling with the wave's energy. Their pliancy allows large changes in the geometry of the working surface, which can be used to tune the converter for specific wave conditions and to protect it from excessive loads in extreme conditions. A submerged converter may be positioned either on the seafloor or in midwater. In both cases, the converter is protected from water impact loads which can occur at the [[free surface]]. Wave loads also diminish in [[Nonlinear system|non-linear]] proportion to the distance below the free surface. This means that by optimizing depth, protection from extreme loads and access to wave energy can be balanced. === Floating in-air converters === [[File:Wellenkraftwerk.JPG|thumb|Wave power station using a pneumatic chamber]] [[File:Wave power station.gif|alt=Simplified design of wave power station|thumb|upright|Simplified design of wave power station]] Floating in-air converters potentially offer increased reliability because the device is located above the water, which also eases inspection and maintenance. Examples of different concepts of floating in-air converters include: * roll damping energy extraction systems with turbines in compartments containing sloshing water * horizontal axis pendulum systems * vertical axis pendulum systems === Submerged wave energy converters === In early 2024, a fully submerged wave energy converter using point absorber-type wave energy technology was approved in Spain.<ref name=InterestingEngineering_20240418/> The converter includes a buoy that is moored to the bottom and situated below the surface, out of sight of people and away from storm waves.<ref name=InterestingEngineering_20240418>{{cite news |last1=Paleja |first1=Ameya |title=Spain set to get table-top-like submerged sea wave energy converter |url=https://interestingengineering.com/energy/spain-submerged-wave-energy-converter |work=Interesting Engineering |date=18 April 2024 |archive-url=https://web.archive.org/web/20240422020251/https://interestingengineering.com/energy/spain-submerged-wave-energy-converter |archive-date=22 April 2024 |url-status=live }}</ref>
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